Wind turbine blades of fibre-reinforced polymer are usually manufactured as shell parts in moulds, where the top side and the bottom side of the blade profile (typically the pressure side and suction side, respectively) are manufactured separately by arranging glass fibre mats in each of the two mould parts. Afterwards, the two halves are glued together, often by means of internal flange parts. Glue is applied to the inner face of the lower blade half before the upper blade half is lowered thereon. Additionally, one or two reinforcing profiles (beams) are often attached to the inside of the lower blade half prior to gluing to the upper blade half.
The shell parts for the wind turbine blade are typically manufactured as fibre composite structures by means of VARTM (vacuum assisted resin transfer moulding), where liquid polymer, also called resin, is filled into a mould cavity, in which fibre material priorly has been inserted, and where a vacuum is generated in the mould cavity, hereby drawing in the polymer. The polymer can be thermoset plastic or thermoplastics.
Vacuum infusion or VARTM is a process used for moulding fibre composite mouldings, where uniformly distributed fibres are layered in one of the mould parts, the fibres being rovings, i.e. bundles of fibre bands, bands of rovings, or mats, which are either felt mats made of individual fibres or woven mats made of fibre rovings. The second mould part is often made of a resilient vacuum bag, and is subsequently placed on top of the fibre material. By generating a vacuum, typically 80% to 95% of the total vacuum, in the mould cavity between the inner side of the mould part and the vacuum bag, the liquid polymer can be drawn in and fill the mould cavity with the fibre material contained herein. So-called distribution layers or distribution tubes, also called inlet channels, are used between the vacuum bag and the fibre material in order to obtain as sound and efficient a distribution of polymer as possible. In most cases, the polymer applied is polyester or epoxy, and the fibre reinforcement is most often based on glass fibres or carbon fibres.
It is commonly known that moulds for making large articles, such as wind turbine blades, and consisting of two mould parts are closed about a longitudinal hinge line, where the hinges are passive, i.e. a crane is used to lift one of the mould parts about the hinge line for closure and opening of the mould. When making wind turbine blades, the mould is closed so as to glue two blade shell halves together, said shell halves being produced in separate mould parts.
Wind turbine blades have become increasingly longer over the years and blades of more than 60 metres are mass-produced, which means that mould assemblies for moulding such blades also have become increasingly larger. This results in problems in regard to the mould assemblies used, since the mould part rotating with respect to the other one during closure of the mould assembly reaches a very great height during the rotary motion, which may entail that the height of the ceiling in the halls where the blades are manufactured must be very great. This means that the halls become more expensive to build, or that the ceilings in existing halls have to be raised, which of course also results in higher financial costs. Furthermore, transport of these large composite structures is problematic.
Therefore, it has been proposed to separate wind turbine blades into two or more separate blade sections and then assemble the blades at the erection site of a wind turbine. Thereby, it is possible to manufacture the separate blade sections in smaller moulds and it is less problematic to transport the much smaller blade sections. An example of such a blade is described in WO 06/103307.
As another example, US2003/0138290A1 discloses a butt connection for joining a first hollow profile member and a second hollow profile member, whereby a wind turbine blade can be formed. The first hollow profile member and the second hollow profile member are joined by means of a multiplicity of straps bridging over the joint between them, and by each of the straps having one end fixed to the first hollow profile member and the other end fixed to the second hollow profile member.
As yet another example, EP1584817A1 discloses a wind turbine blade transversely subdivided into two or more independent modules, where each modules comprises an internal longitudinal reinforcement structure, that at the end of the internal longitudinal reinforcement structure comprises lugs, such that the modules that be connected to each other. Subsequently, a cuff is arranged on top of the lugs so as to provide an aerodynamic surface.
However, the dividable wind turbine blade disclosed in US2003/0138290A1 is not aerodynamically efficient and will generate noise as a number of straps are arranged externally to the wind turbine blades aerodynamically profile, and thus the straps will interfere and disturb the flow around the dividable wind turbine blade. The dividable wind turbine blade disclosed in EP1584817A1 and WO06/103307 resolves these problems. However the problems are resolved by introducing an internal reinforcement structure along the longitudinal direction of the dividable wind turbine blade, and thus the problems remains unsolved for a wind turbine blade without a thoroughgoing internal reinforcement structure. Furthermore, the dividable blade is encumbered with the disadvantage that the joint is arranged internally of the blade shell, which means that one either has to climb into the blade shell in order to assemble the blade or that the blade has to be covered with an additional surface cover after assembly. Also, the joint is provided only in a central part of the cross section. Thus, the joint may cause an unbalance or internal stress boundaries in the cross section or transverse direction of the blade.